Two-stage projector architecture

ABSTRACT

A light projection system projects an image comprising a matrix of light pixels having modulated luminance. The projection system includes a first imager configured to modulate a light band on a pixel-by-pixel basis proportional to gray scale values provided for each pixel of the image to provide a first output matrix. A second imager is positioned and configured to receive the first output matrix of modulated pixels of light and modulate the individual modulated pixels of light from said first imager on a pixel-by-pixel basis proportional to a second gray scale value provided for each pixel of the image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/US03/37978, filed Nov. 26, 2003, which waspublished in accordance with PCT Article 21(2) on Jun. 17, 2004 inEnglish and which claims the benefit of U.S. provisional patentapplication Ser. No. 60/430,997, filed Dec. 4, 2002 and U.S. provisionalpatent application Ser. No. 60/430,819, filed Dec. 4, 2002.

FIELD OF THE INVENTION

The present invention relates to a display system having a two-stageprojector architecture.

BACKGROUND

Liquid crystal displays (LCDs), and particularly liquid crystal onsilicon (LCOS) systems using a reflective light engine or imager, arebecoming increasingly prevalent in imaging devices such as rearprojection television (RPTV). In an LCOS system, projected light ispolarized by a polarizing beam splitter (PBS) and directed onto a LCOSimager or light engine comprising a matrix of pixels. Throughout thisspecification, and consistent with the practice of the relevant art, theterm pixel is used to designate a small area or dot of an image, thecorresponding portion of a light transmission, and the portion of animager producing that light transmission.

Each pixel of the imager modulates the light incident on it according toa gray-scale factor input to the imager or light engine to form a matrixof discrete modulated light signals or pixels. The matrix of modulatedlight signals is reflected or output from the imager and directed to asystem of projection lenses which project the modulated light onto adisplay screen, combining the pixels of light to form a viewable image.In this system, the gray-scale variation from pixel to pixel is limitedby the number of bits used to process the image signal. The contrastratio from bright state (i.e., maximum light) to dark state (minimumlight) is limited by the leakage of light in the imager.

One of the major disadvantages of existing LCOS systems is thedifficulty in reducing the amount of light in the dark state, and theresulting difficulty in providing outstanding contrast ratios. This is,in part, due to the leakage of light, inherent in LCOS systems.

In addition, since the input is a fixed number of bits (e.g., 8, 10,etc.), which must describe the full scale of light, there tend to bevery few bits available to describe subtle differences in darker areasof the picture. This can lead to contouring artifacts.

One approach to enhance contrast in LCOS in the dark state is to use aCOLORSWITCH™ or similar device to scale the entire picture based uponthe maximum value in that particular frame. This improves some pictures,but does little for pictures that contain high and low light levels.Other attempts to solve the problem have been directed to making betterimagers, etc. but these are at best incremental improvements.

What is needed is a projection system that enhances the contrast ratiofor video images, particularly in the dark state, and reduces contouringartifacts.

SUMMARY

The present invention provides a projection system that providesimproved contrast and contouring of a light signal on a pixel-by-pixelbasis using a two-stage projection architecture, thus improving allvideo pictures. In an exemplary embodiment of the present invention,this projection system includes a first imager configured to modulate alight band on a pixel-by-pixel basis proportional to gray scale valuesprovided for each pixel of the image to provide a first output matrix. Asecond imager is positioned and configured to receive the first outputmatrix of modulated pixels of light and modulate the individualmodulated pixels of light from said first imager on a pixel-by-pixelbasis proportional to a second gray scale value provided for each pixelof the image. Each pixel of the second imager provides a light output ofintensity proportional to a modulated light output of a correspondingpixel in the first imager and a selected gray scale value for that pixelin the second imager.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to accompanyingfigures of which:

FIG. 1 shows a block diagram of an LCOS projection system with atwo-stage projection architecture according to an exemplary embodimentof the present invention;

FIG. 2 shows an exemplary lens relay system for the projection system ofFIG. 1;

FIG. 3 shows light leakage in the projection system of FIG. 1;

FIG. 4 shows a block diagram of an LCOS projection system with atwo-stage. projection architecture according to an alternative exemplaryembodiment of the invention;

FIG. 5 shows an exemplary relay lens system for the projection system ofFIG. 4,

FIG. 6 shows the calculated ensquared energy for the relay lens systemof FIG. 5; and

FIG. 7 shows the distortion due to field curvature for the relay lenssystem of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a projection system, such as for atelevision display, with enhanced contrast ratio and reduced contouring.In an exemplary LCOS system, illustrated in FIG. 1, white light 1 isgenerated by a lamp 10. Lamp 10 may be any lamp suitable for use in anLCOS system. For example a short-arc mercury lamp may be used. The whitelight 1 enters an integrator 20, which directs a telecentric beam ofwhite light 1 toward the projection system 30. The white light 1 is thenseparated into its component red, green, and blue (RGB) bands of light2. The RGB light 2 may be separated by dichroic mirrors (not shown) anddirected into separate red, green, and blue projection systems 30 formodulation. The modulated RGB light 2 is then recombined by a prismassembly (not shown) and projected by a projection lens assembly 40 ontoa display screen (not shown).

Alternatively, the white light 1 may be separated into RGB bands oflight 2 in the time domain, for example, by a color wheel (not shown),and thus directed one-at-a-time into a single LCOS projection system 30.

An exemplary LCOS projection system 30 is illustrated in FIG. 1, using atwo-stage projection architecture according to the present invention.The monochromatic RGB bands of light 2 are sequentially modulated by twodifferent imagers 50, 60 on a pixel-by-pixel basis. The RGB bands oflight 2 comprise a p-polarized component 3 and an s-polarized component4 (shown in FIG. 3). These RGB bands of light 2 enter a first surface 71a of a first PBS 71 and are polarized by a polarizing surface 71 pwithin the first PBS 71. The polarizing surface 71 p allows thep-polarized component 3 of the RGB bands of light 2 to pass through thefirst PBS 71 to a second surface 71 b, while reflecting the s-polarizedcomponent at an angle, away from the projection path where it passes outof first PBS 71 through fourth surface 71 d. A first imager 50 isdisposed beyond the second surface 71 b of the first PBS 71 opposite thefirst face 71 a, where the RGB bands of light enter first PBS 71. Thep-polarized component 3, which passes through the PBS 71, is thereforeincident on the first imager 50.

In the exemplary embodiment, illustrated in FIG. 2, first imager 50 isan LCOS imager comprising a matrix of polarized liquid crystalscorresponding to the pixels of the display image (not shown). Thesecrystals transmit light according to their orientation, which in turnvaries with the strength of an electric field created by a signalprovided to the first imager 50. The imager pixels modulate thep-polarized light 3 on a pixel-by-pixel basis proportional to a grayscale value provided to the first imager 50 for each individual pixel.As a result of the modulation of individual pixels, the first imager 50provides a first light matrix 5, comprising a matrix of pixels ordiscreet dots of light. First light matrix 5 is an output of modulateds-polarized light reflected from the first imager 50 back through secondsurface 71 b of first PBS 71, where it is reflected by a polarizingsurface 71 p at an angle out of the first PBS 71 through a third surface71 c. Each pixel of the first light matrix 5 has an intensity orluminance proportional to the individual gray scale value provided forthat pixel in first imager 50.

The first light matrix 5 of s-polarized light is reflected by the PBS 71through a relay lens system 80, which provides 1-to-1 transmission ofthe first light matrix 5. In an exemplary embodiment, illustrated inFIG. 2, relay lens system 80 comprises a series of aspherical andacromatic lenses configured to provide low distortion of the image beingtransmitted with a magnification of 1, so that the output of each pixelin the first imager 50 is projected onto a corresponding pixel of thesecond imager 60.

As shown in FIG. 2, exemplary relay lens system 80 comprises a firstaspheric lens 81 and a first acromatic lens 82 between the first PBS 71and the focal point of the lens system or system stop 83. Between thesystem stop 83 and the second imager 72, lens system 80 comprises asecond acromatic lens 84 and a second aspheric lens 85. First asphericlens 81 has a first surface 81 a and second surface 81 b which bend thediverging light pattern from the first PBS 71 into a light patternconverging toward the optical axis of lens system 80. First acromaticlens 82 has a first surface 82 a, a second surface 82 b, and a thirdsurface 82 c, which focus the converging light pattern from the firstaspheric lens 81 onto the system stop 83. At the system stop 83, thelight pattern inverts and diverges. The second acromatic lens 84, whichhas a first surface 84 a, a second surface 84 b, and a third surface 84c, is a mirror image of first acromatic lens 82 (i.e., the same lensturned backward such that first surface 84 a of second acromatic lens 84is equivalent to third surface 82 c of first acromatic lens 82 and thirdsurface 84 c of second acromatic lens 84 is equivalent to first surface82 a of first acromatic lens 82). The surfaces 84 a, 84 b, and 84 c ofsecond acromatic lens 84 distribute the diverging light pattern onto thesecond aspherical lens 85. The second aspherical lens 85, which has afirst surface 85 a and a second surface 85 b, is a mirror image of thefirst aspherical lens 81. Surfaces 85 a and 85 b bend the light patternto converge to form an inverted image on the second imager 72 that has aone-to-one correspondence to the object or matrix of pixels from thefirst imager 50. The surfaces of relay lens system 80 are configured towork with the imagers 50, 60 and PBS's 71, 72 to achieve the one-to-onecorrespondence of the pixels of first imager 50 and second imager 60. Asummary of the surfaces of an exemplary two-stage projection system 30are provided in Table 1, and aspheric coefficients for surfaces 81 a, 81b, 85 a, and 85 b are provided in Table 2. Various modifications can bemade to this exemplary projection system based on such factors as: cost,size, luminance levels, and other design factors. In relay lens system80 acromatic lenses 82 and 84 are equivalent and aspherical lenses 81and 85 are equivalent. Therefore, fewer unique parts are requiredproviding manufacturing efficiencies and reduced cost.

TABLE 1 (dimensions in millimeters) Device surface type Radius thicknessglass diameter conic 50 object std Infinity 11.25436 0 71 2nd (71b) stdInfinity 28 SF2  24.07539 0 71 3rd (71c) std Infinity 11.44304 29.597820 81 1st (81a) evenasph 45.72373 11.60359 BAK2 40 −0.941321 81 2nd (81b)evenasph −29.74398 1.061985 40 −2.300802 82 1st (82a) std 16.452019.507266 BAK2 28 −0.003454099 82 2nd (82b) std 696.8212 6.993905 SF15 280 82 3rd (82c) std 10.75055 6.389217 14 0 83 stop std Infinity 6.38921710.09268 0 84 1st (84a) std −10.75055 6.993905 SF15 14 0 84 2nd (84b)std −696.8212 9.507266 BAK2 28 0 84 3rd (84c) std −16.45201 1.061985 28−0.003454099 85 1st (85a) evenasph 29.74398 11.60359 BAK2 40 −2.30080285 2nd (85b) evenasph −45.72373 11.44304 40 −0.941321 72 1st (72a) stdInfinity 28 SF2  31.9247 0 72 2nd (72b) std Infinity 11.25436 25.06428 060 image std Infinity 20.44114 0

TABLE 2 coefficient on: surfaces 81a, 85b surfaces 81b, 85a r²−2.5672672e−005 −2.5672139e−005 r⁴ −3.6464646e−007 −4.6458644e−007 r⁶−1.3360629e−009 −5.3232556e−010 r⁸  2.2079531e−012  9.3458426e−013 r¹⁰ 4.0274312e−019 −2.9875722e−019 r¹²  3.2408025e−022  8.6192792e−022 r¹⁴−4.2302691e−024  5.022829e−024 r¹⁶ −1.3350251e−026  1.9281194e−026

After the first light matrix 5 leaves the relay lens system 80, itenters into a second PBS 72 through a first surface 72 a. Second PBS 72has a polarizing surface 72 p that reflects the s-polarized first lightmatrix 5 through a second surface 72 b onto a second imager 60. In theexemplary embodiment, illustrated in FIG. 2, second imager 60 is an LCOSimager which modulates the previously modulated first light matrix 5 ona pixel-by-pixel basis proportional to a gray scale value provided tothe second imager 60 for each individual pixel. The pixels of the secondimager 60 correspond on a one-to-one basis with the pixels of the firstimager 50 and with the pixels of the display image. Thus, the input of aparticular pixel (i,j) to the second imager 60 is the output fromcorresponding pixel (i,j) of the first imager 50.

The second imager 60 then produces an output matrix 6 of p-polarizedlight. Each pixel of light in the output matrix 6 is modulated inintensity by a gray scale value provided to the imager for that pixel ofthe second imager 60. Thus a specific pixel of the output matrix 6 (i,j)would have an intensity proportional to both the gray scale value forits corresponding pixel (i,j)₁ in the first imager and its correspondingpixel (i,j)₂ in the second imager 60.

The light output L of a particular pixel (i,j) is given by the productof the light incident on the given pixel of first imager 50, the grayscale value selected for the given pixel at first imager 50, and thegray scale value selected at second imager 60:L=L0×G1×G2

L0 is a constant for a given pixel (being a function of the lamp 10, andthe illumination system.) Thus, the light output L is actuallydetermined primarily by the gray scale values selected for this pixel oneach imager 50, 60. For example, normalizing the gray scales to 1maximum and assuming each imager has a very modest contrast ratio of200:1, then the bright state of a pixel (i,j) is 1, and the dark stateof pixel (i,j) is 1/200 (not zero, because of leakage). Thus, the twostage projector architecture has a luminance range of 40,000:1.Lmax=1×1=1;Lmin=0.005×0.005=0.000025

The luminance range defined by these limits gives a contrast ratio of1/0.000025:1, or 40,000:1. Importantly, the dark state luminance for theexemplary two-stage projector architecture would be only aforty-thousandth of the luminance of the bright state, rather than onetwo-hundredth of the bright state if the hypothetical imager were usedin an existing single imager architecture. As will be understood bythose skilled in the art, an imager with a lower contrast ratio can beprovided for a considerably lower cost than an imager with a highercontrast ratio. Thus, a two-stage projection system using two imagerswith a contrast ratio of 200:1 will provide a contrast ratio of40,000:1, while a single-stage projection system using a much moreexpensive imager with a 500:1 ratio will only provide a 500:1 contrast.Also, a two-stage projection system with one imager having a 500:1contrast ratio and an inexpensive imager with a 200:1 ratio will have asystem contrast ratio of 100,000:1. Accordingly, a cost/performancetrade-off can be performed to create an optimum projection system.

Output matrix 6 enters the second PBS 72 through second surface 72 b,and since it comprises p-polarized light, it passes through polarizingsurface 72 p and out of the second PBS 72 through third surface 72 c.After output matrix 6 leaves the second PBS 72, it enters the projectionlens assembly 40, which projects a display image 7 onto a screen (notshown) for viewing.

FIG. 3 illustrates the reduced leakage of the two-stage projectorarchitecture of the present invention. As described above, thepolarizing surface 71 p of first PBS 71 transmits p-polarized light 3,which leaves the first PBS through the second surface 71 b. Thepolarizing surface 71 p of first PBS 71 reflects s-polarized light 4through the fourth surface 71 d. Even the best PBS will have someleakage, however, and a very small portion of s-polarized light, firstleakage 4′, will be transmitted or leak through the polarizing surface71 p of first PBS 71 and out second surface 71 b to first imager 50.Most of the first leakage 4′ will be inverted by first imager 50 top-polarized light and pass through polarizing surface 71 p and out firstsurface 71 a of first PBS 71 toward light 10. Similarly, a small portionof p-polarized light 3 will be reflected by first imager 50, asp-polarized second leakage 3′. Since second leakage 3′ is p-polarizedlight, most of it will pass back through the polarizing surface 71 p offirst PBS 71 and out first surface 71 a toward light 10.

A small portion of p-polarized first leakage 3′ and p-polarized secondleakage 4″ will be reflected by polarizing surface 71 p of PBS 1 towardsecond PBS 72 as fourth leakage 3″, 4′″. Most of this third leakage 3″,4′″ will pass through the polarizing surface 72 p of second PBS 72 andaway from the image path through fourth surface 72 d, because it isp-polarized light. A small portion of third leakage 3″, 4′″ will bereflected onto second imager 60 as fourth leakage 3′″, 4″″. Most of thefourth leakage 3′″, 4″″ will be inverted to s-polarized light and getreflected back toward first PBS 71. As one skilled in the art willappreciate, each subsequent time that a light leakage through a PBS orreflection is transmitted through a PBS 71, 72, most of the lightleakage is diverted from the image path, such that only a portion of theprevious portion continues along the image path. Thus, each time a lightsignal passes through a PBS, inverted polarized leakage is reduced.

As with the PBS, no imager is perfect, and a small portion of light willbe reflected as s-polarized fifth leakage 8 by pixels of the firstimager 50 with a gray scale value set to the dark state. In sharpcontrast to the p-polarized fourth leakage, most of the s-polarizedfifth leakage 8 will be reflected by polarizing surface 71 p andpolarizing surface 72 p onto second imager 60, because it has the samepolarization as first light matrix 5. When the corresponding pixel ofthe second imager 60 is set to the dark state, the fifth leakage 8 isfurther reduced by the contrast ratio of second imager 60 as describedabove, reducing the light in the dark state to sixth leakage 8′ andenhancing the contrast ratio.

Additional light leakage 9 occurs at the second imager 60. Thisadditional light leakage 9, however is from the first light matrix 5,previously modulated by first imager 71, and would therefore be aportion of a much smaller light signal than the RGB bands 3 incident onthe first imager 71. Accordingly, the two-stage projection architectureof the present invention reduces leakage, both through additionalpolarized filtering and through cumulative contrast ratios of serialimagers 50, 60.

Another advantage of the two-stage projector system according to thepresent invention is that a larger number of bits are available todescribe subtle differences in shading in darker areas of the image,reducing contouring. For example, if eight bits are used to produce agray scale value for each pixel of an imager, then 2⁸ or 256 shades ofgray can be defined. Because two imagers 50, 60 modulate the same pixelof an image sequentially in the present invention, however, 2¹⁶ or65,536 shades of gray can be defined. Thus contouring can be greatlyreduced.

In an alternative embodiment of the invention, as shown in FIGS. 4 and5, a two stage projection system comprises a first stage 100 havingthree first stage imagers 150R, 150B and 150G for modulating red, green,and blue light, respectively, on as pixel-by-pixel basis. Each firststage imager 150R, 150B, 150G provides a matrix of modulatedmonochromatic light pixels 151P. The fist stage imagers rotate thepolarization of the light so that the output matrices 151P arep-polarized light. In the illustrated embodiment, the first stage 100 isconfigured in a COLORQUAD™ architecture, wherein four color-selectivebeam splitters 120 are abutted to form a square pattern and arrangedsuch that they selectively pass or reflect various colors of light sothat each color of light entering the first stage 100 is directed to adifferent first stage imager 150R, 150B, 150G. The COLORQUAD™architecture directs only red light to the red imager 150R, only bluelight to the blue imager 150B, and only green light to the green imager150G. The COLORQUAD™ architecture then directs the three matrices 151Pof modulated monochromatic light pixels out of the first stage 100toward a second stage 200. It should be noted that the various colors oflight each travel the same distance in the first stage 100. Thus thethree colors can be modulated and projected simultaneously.

The embodiment illustrated in FIG. 4, also comprises a second stage 160having three second stage imagers 160R, 160B, 160G arranged in aCOLORQUAD™ architecture, which again divides light into three differentcolors: red, blue and green and directs them to the three second stageimagers 160R, 160B, 160G. The second stage imagers 160R, 160B, 160Gmodulate light output from the first stage imagers 150R, 150B, 150G onas pixel-by-pixel basis, to form twice modulated matrices ofmonochromatic light pixels 201. These twice modulated matrices ofmonochromatic light pixels 201 are directed by the COLORQUAD™architecture into a projection lens system (not shown) which projectsthem onto a screen (not shown) to form a viewable color image.

An alterative exemplary relay lens system 380 is disposed between thefirst stage 150 and the second stage 160 to project the output of thepixels of the first stage imagers 150R, 150B, 150G onto correspondingpixels of the second stage imagers 160R, 160 b, 160G, such that aparticular pixel of the output of the second stage will comprise a redpixel, a blue pixel and a green pixel, each being modulated twice (onceby a first stage imager 150R, 150B, 150G and once by a correspondingsecond stage imager 160R, 160B, 160G).

The COLORQUAD™ architecture should receive only s-polarized light.Accordingly, s-polarixed light 101 is introduced into the first stage100, where the s-polarized light 101 is divided into three colors whichare individually and simultaneously modulated by the three imagers 150R,150B, and 150G. Since the COLORQUAD™ architecture of the second stage200 should receive s-polarized input, and the matrices of modulatedmonochromatic pixels of light 151 from the first stage 100 isp-polarized light, the polarization must be rotated between the firststage 100 and the second stage 200. Accordingly, a half-wave plate 383is disposed between the first stage 100 and the second stage 200 in theembodiment illustrated in FIGS. 4 and 5. The most effective location forthe half-wave plate 383 is at the aperture or lens stop of the relaylens system.

As shown in FIG. 5, the alternate exemplary relay lens system 380comprises a first aspheric lens 381 and a first acromatic lens 382between the first stage 100 and the lens stop or system stop. Betweenthe system stop and the second stage 200, the alternative exemplary lenssystem 380 comprises a second acromatic lens 384 and a second asphericlens 385. First aspheric lens 381 has a first surface 381 a and secondsurface 381 b which bend the diverging light pattern from the firststage 100 into a light pattern converging toward the optical axis oflens system 380. First acromatic lens 382 has a first surface 382 a, asecond surface 382 b, and a third surface 382 c, which focus theconverging light pattern from the first aspheric lens 381 onto thesystem stop. At the system stop, the light pattern inverts and diverges.The second acromatic lens 384, which has a first surface 384 a, a secondsurface 384 b, and a third surface 384 c, has the same configuration butan opposite orientation of first acromatic lens 382 (i.e., the same lensturned backward such that first surface 384 a of second acromatic lens384 is equivalent to third surface 382 c of first acromatic lens 382 andthird surface 384 c of second acromatic lens 384 is equivalent to firstsurface 382 a of first acromatic lens 382). The surfaces 384 a, 384 b,and 384 c of second acromatic lens 384 distribute the diverging lightpattern onto the second aspherical lens 385. The second aspherical lens385, which has a first surface 385 a and a second surface 385 b, has thesame configuration, but opposite orientation of the first asphericallens 381. Surfaces 385 a and 385 b bend the light pattern to converge toform an inverted image on the second stage 200 that has a one-to-onecorrespondence to the object or matrix of pixels from the first stage100. The surfaces of relay lens system 380 are configured to work withthe imagers 150R, 150B, 150G, 160R, 160B, 160G and COLORQUAD™architecture to achieve the one-to-one correspondence of the pixels offirst stage imagers 150R, 150B, 150G and the second imager 160R, 160B,160G. A summary of the surfaces of an alternative exemplary relay lenssystem 380 is provided in Table 3, and aspheric coefficients forsurfaces 381 a, 381 b, 385 a, and 385 b are provided in Table 4. Whilered imagers 150R and 160R are shown, the relay lens system is the samefor the blue and green imagers. Various modifications can be made tothis alternative exemplary relay lens system 380 based on such factorsas: cost, size, luminance levels, and other design factors. Inalternative exemplary relay lens system 380, acromatic lenses 382 and384 are equivalent and aspherical lenses 381 and 385 are equivalent.Therefore, fewer unique parts are required providing manufacturingefficiencies and reduced cost.

TABLE 3 (dimensions in millimeters) Surface Type Radius Thickness GlassDiameter Conic 150R Standard Infinity 5 20.4 0 120 Standard Infinity 22SF2 35.6 0 120 Standard Infinity 2.01 BK7 35.6 0 120 Standard Infinity27 SF2 43.42 0 120 Standard Infinity 3.39 BK7 43.42 0 120 StandardInfinity 15.35734 34.44785 0 381a Evenasph 54.31735 11.64581 BAK241.60527 −.300586 381b Evenasph −53.66137 16.97665 41.49939 −2.189343382a Standard 16.10644 10.20515 BAK2 28.1681 −0.03218522 382b Standard51.12306 4.214673  SF15 23.92413 0 382c Standard 12.39113 9.16019316.74992 0.3226084 STO Standard Infinity 1.1 BK7 13.61922 0 383 StandardInfinity 9.160193 14.0642 0 384a Standard −12.39113 4.214673  SF1516.74992 0.3226084 384b Standard −51.12306 10.20515 BAK2 23.92413 0 384cStandard −16.10644 16.97665 28.16813 −0.03218522 385a Evenasph 53.6613711.64581 BAK2 41.49939 −2.189343 385b Evenasph −54.31735 15.3573441.60527 −1.300586 120 Standard Infinity 3.39 BK7 34.44785 0 120Standard Infinity 27 SF2 43.42 0 120 Standard Infinity 2.01 BK7 43.42 0120 Standard Infinity 22 SF2 35.6 0 120 Standard Infinity 5 35.6 0 160RStandard Infinity 20.4 0

TABLE 4 Coefficient Surface 381a: Surface 381b: Surface 385a: Surface385b: on: Evenasph Evenasph Evenasph Evenasph r²  7.2863779e−005 6.5067317e−005  6.5067317e−005  7.2863779e−005 r⁴  3.8342926e−007 3.7276989e−007  3.7276989e−007  3.8342926e−007 r⁶ −1.6715824e−009−1.3168408e−009 −1.3168408e−009 −1.6715824e−009 r⁸  6.3658429e−012 2.828627e−012  2.828627e−012  6.3658429e−012 r¹⁰ −2.3418598e−015 1.0954082e−014  1.0954082e−014 −2.3418598e−015 r¹² −2.1109997e−018−9.2267507e−019 −9.2267507e−019 −2.1109997e−018 r¹⁴  3.4975822e−020−6.4427989e−020 −6.4427989e−020  3.4975822e−020 r¹⁶ −1.2083771e−022 1.723144e−023  1.723144e−023 −1.2083771e−022

The ensquared energy and field curvature distortion were calculated forthe alternative exemplary relay lens system 380 using ZEMAX™ software.As shown in FIG. 6, at least about fifty percent (50%) of the lightenergy from a particular pixel on a first stage imager 150R, 150B, 150Gis focused onto a twelve micron square (e.g., the corresponding pixel ofa second stage imager 160R, 160B, 160G). As shown in FIG. 7, thedistortion due to field curvature of the alternative exemplary relaysystem 380 is less than about 0.5 percent.

The foregoing illustrates some of the possibilities for practicing theinvention. Many other embodiments are possible within the scope andspirit of the invention. For example, while the forgoing description andthe Figures are directed to an LCOS imager, a similar system using adigital light processing (DLP) imager is also contemplated within thescope of the invention. It is, therefore, intended that the foregoingdescription be regarded as illustrative rather than limiting, and thatthe scope of the invention is given by the appended claims together withtheir full range of equivalents.

1. A two-stage projection system comprising: a first stage having one ormore imagers for modulating light on a pixel-by-pixel basis; a secondstage having one or more imagers for modulating light on apixel-by-pixel basis; a relay lens system projecting the output of thepixels of the first stage imagers onto corresponding pixels of thesecond stage imagers; and a half-wave plate disposed between the firststage and the second stage.
 2. The two-stage imager of claim 1 whereinthe first stage comprises three imagers for modulating red, blue, andgreen light, respectively and an architecture configured to direct lightof the desired color to each imager.
 3. The two-stage imager of claim 2wherein the second stage comprises three imagers for modulating red,blue, and green light, respectively and an architecture configured todirect light of the desired color to each imager.
 4. The two-stageimager of claim 1 wherein the half-wave plate is disposed at the systemstop of the relay lens system.
 5. The two-stage imager of claim 1wherein the relay lens system ensquares at least about 50 percent of theenergy from a particular pixel of a first stage imager onto thecorresponding pixel of a second stage imager.
 6. The two-stage imager ofclaim 1 wherein the relay lens system provides a distortion due to fieldcurvature of less than about 0.5 percent.